Electroless plating baths typically comprise a metal salt, a chelant for the metal species, a reducing agent for the metal and stabilizers to retard the tendency of the reducing agent to promote reduction and deposition of the metal, e.g. on indiscriminate surfaces or in the bulk solution. In nickel electroless plating baths of the prior art, the nickel salt is typically associated with a divalent counterion such as sulfate. It has been discovered that sulfate ions create problems in treating spent electroless nickel plating baths. For instance, not only are sulfate ions not environmentally acceptable in many effluent streams, but sulfate ions are difficult to separate from desirable polyvalent anions such as chelant species.
Spent plating baths are traditionally treated by adding reducing agent such as sodium borohydride to precipitate elemental metal; residual soluble metal is precipitated with strong complexing agents, e.g. dithiocarbamate. Metal chelant complexes can be decomposed by oxidizing chelants, e.g. with peroxides, hypochlorates or other oxidizing acids.
Because of the difficulties in treating spent plating baths, disposal in landfills is often a method of choice for disposing of spent plating solutions or metal sludge precipitate from plating baths. Typically sulfate is removed from plating solutions by lime treatment forming gypsum contaminated with metal, e.g. nickel, which is not acceptable for disposal in a growing number of landfills. Moreover, metal recyclers often prefer to avoid spent electroless nickel solutions because of the high phosphorus content.
U.S. Pat. No. 5,039,497 discloses methods of removing copper from sulfate solutions using oximes. Cognis, Inc. (Santa Rosa, Calif.) has disclosed that such an extraction process can be used to treat copper and nickel electroless solutions to reduce the metal content producing a solution suitable for disposal, e.g. by sewering. Such solvent extraction methods have not been enthusiastically adopted for treating plating baths comprising copper complexed with EDTA, in part because common commercial extractants such as oximes are not especially effective in extracting metals from complexes. For instance, copper is effectively extracted from EDTA at a pH in the range of 12-12.5, the same pH used for electroless plating; simultaneous plating and extraction is not desirable. Another disadvantage of the proposed extraction is that, because nickel is invariably associated with cobalt, which irreversibly binds to oximes, the extractant is progressively poisoned.
Cardotte in U.S. Pat. No. 4,895,661 discloses the use of hyperfiltration membranes to process copper electroless plating solutions, e.g. to concentrate for re-use salts of EDTA. Such membranes are more permeable to monovalent anions such as formate than polyvalent anions such as tartrate which are often used as chelants in electroless metal plating baths. It has been found that an undesirably high level of metal salts or chelated complexes permeate such membranes when treating plating bath purge streams. Such metal-containing permeate streams are typically unsuitable for waste disposal in many places.
An object of this invention is to provide apparatus and methods for removing polyvalent oxized by-products of reducing agents from metal solutions in streams essentially devoid of metal as to allow environmentally acceptable disposal.